The invention relates generally to deployable reflector antennas, and in particular to a deployable reflectarray antenna system.
Reflector antennas have a long history of development for various uses in space. The need for antennas with ever larger collection surface areas led to the development of deployable antennas with relatively small stowed footprints that would fit within the limited dimensions of launch vehicle payloads. The bulk of these deployable reflector antennas are parabolic dish structures that are stowed and deployed by a variety of often complex mechanisms. At higher frequencies and larger deployed antenna diameters, it becomes more and more difficult to attain and to thereafter maintain required antenna surface tolerances. Deviations from the desired shape reduce antenna gain and increase undesired side lobes.
Phased array antennas offer a number of benefits for space operations. By controlling the phase of the transmitted or received electromagnetic radiation a great deal of control can be exerted over the resulting beam pattern. Many desirable traits such as the reduction of side lobes and cross-polarized fields in the beam pattern can be designed into even the simplest of phased arrays. By incorporating controllable phase shifters into the feed structure, the beam pattern can be adapted during operation to suit a variety of needs. The planar nature of these arrays can enable a fairly simple deployment mechanism as well. Since the beam pattern primarily depends on the phase of each array element, the surface tolerance of the deployed structure becomes less of an issue. An analysis comparing the characteristics of phased array antennas with reflector antennas can be found in Wang, H.S.C., “A comparison of the performance of reflector and phased-array antennas under error conditions”, 1991 IEEE Aerospace Applications Conference Digest, p 4/1-4, 1991.
A reflectarray antenna combines some of the best features of reflector and array antennas. Basically a microstrip reflectarray antenna consists of a flat array of microstrip patches or dipoles printed on a thin dielectric substrate. A feed antenna illuminates the array. The individual microstrip patches are designed to scatter the incident field with the proper phase required to form a planar phase front when a feed is placed at its focus similar to a parabolic reflector. These flat reflectarray antennas can be produced at relatively low cost, with high gain and are particularly effective at high frequencies. Additional details of microstrip reflectarray antennas can be found in Pozar, D. M. et al, “Design of Millimeter Wave Microstrip Reflectarrays,” IEEE Trans. Of Antennas and Propagation, Vol. 45, No. 2, February 1997.
Although the losses from microstrip reflectarray antennas are typically less than those of a phased array, they are still greater than those of a fixed aperture parabolic reflector. For example, etching tolerances can introduce phase errors in the reflectors and the dielectric substrate can attenuate the signal. This results in lower aperture efficiencies and lower gains than are possible with a simple fixed aperture reflector of the same surface area.
Reflectarrays can be less expensive to manufacture than phased arrays or parabolic dishes, and by design they offer a degree of control over the beam pattern superior to that of a parabolic reflector. It is desirable to leverage the established design methods for reflectarrays and the mechanical advantages they offer a deployable structure. With deployable structures lower aperture efficiencies are compensated for with higher gain from increased antenna collection area. If a simple deployment mechanism were available, large aperture reflectarray antennas should become commercially successful for space applications and for terrestrial applications where a small stowed configuration is desirable. This is the intent of the present invention.